Process and apparatus for flash spinning of fibrillated plexifilamentary material



R D. ANDERSON ETAL 3,227,794 PROCESS AND APPARATUS FOR FLASH SPINNING OF FIBRILLATED Jan. 4, 1966 4 Sheets-Sheet 1 Filed Sept. 13, 1963 wzoN 2:2 5: :33:

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RONALD DEAN ANDERSON BY JOHN EMILIO ROMANO ATTORNEY R. D. ANDERSON ETAL 3,227,794 PROCESS AND APPARATUS FOR FLASH SPINNING OF FIBRILLATED PLEXIFILAMENTARY MATERIAL 4 Sheets-Sheet 2 Jan. 4, 1966 Filed Sept. 13, 1963 :5 32;: 5:3 fi I I I l I I IIIII l I I l 1 I I I 1 IIIIIIIIIIII: III V I I I I I II III II a 3523 3E8 I 2525 :23: 22 E5 25 I I v 5:3 3528 1 I IIIIJ 23$: s a 2 EETF T IIIIL r 2 M 3 Q z 5:: I 00 2 2 $25: 3%; 22; 00 g f a a. N 7; Q M w 2; 2... T $.52

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PHASE PHASE BOUNDARIES FOR LINEAR POLYETHYLENHM. l.= 0451) AND C TRICHLOROFLUOROMETHANE SYSTEM i m 209 Logo mp0 1490 Lego l 0 K PRESSURE, PSIA INVENTORS F I G. 5 RONALD DEAN ANDERSON 4mm EMILIO ROMANO ATTORNEY Jan. 4, 1966 R D. ANDERSON ETAL 3,227,794 PROCESS AND APPARATUS FUR FLASH SPINNING OF FIBRILLATED PLEXIFILAMENTARY MATERIAL 4 Sheets-Sheet 4 Filed Sept. 13, 1965 8 mm m WW A R m w M m M DE L A N H 00 n RJ M 4 :5 23m 28 N. 2 :5 m2 mofiw 2: E22 l\00 Q 3% n2 muss: 252 55; E m V 2 BY ywwaw ATTORNEY United States Patent PROCESS AND APPARATUS FOR FLASH SPEN- NLIG OF FIBPJLLATED PLEXIFEAMEN- TARY MATERIAL Ronald Dean Andersen, Richmond, Va., and John Emilio Romano, Hockessin, Deh, assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Sept. 13, 1963, Ser. No. 308,845 26 Claims. (Cl. 264-405) This application is a continuation-in-part of our copending application Ser. No. 239,674, filed Nov. 23, 1962, now abandoned.

This invention relates to an improved process and apparatus for dissolving polymer and spinning fibrillated strands.

In the known art of spinning fibers from a solution it is common to control the denier of the spinning thread line by spinning the solution through a volumetric pump. Although such a spinning system provides filaments with very uniform weight per unit length, the system is often forced to operate with great pressure variations within the piping system. For example, if the polymer does not dissolve completely, gel particles may collect on the solution filters and extreme pressures may be periodically developed. Then unless a new filter is put into service by opening the proper valves an excessive pressure drop may occur. Although such pressure changes are not a severe problem with solutions that are homogeneous at all spinning pressures, they can be extremely serious from the standpoint of quality control with solutions which tend to form two liquid phases when the pressure drops below a certain level. A spinning solution containing a dissolved polyolefin such as polyethylene is particularly susceptible to phase separations of such a nature.

In United States Patent 3,081,519 to Blades and White, filed January 31, 1962, there is described a novel and useful multi-fibrous, yarn-like strand formed by flash spinning a homogeneous solution of a fiber-forming polymer in a liquid which is a non-solvent for the polymer below its normal boiling point, at a temperature above the normal boiling point of the liquid, and at autogenous pressures or greater into a medium of lower temperature and substanitally lower pressure. The vaporizing liquid within the extrudate forms bubbles, breaks through confining walls, and cools the extrudate, causing solid polymer to form therefrom. The resulting multi-fibrous yarn-like strand has an internal fine structure or morphology characterized as a three-dimensional integral plexus consisting of a multitude of essentially longitudinally extended interconnecting random length fibrous elements, referred to as film-fibrils, which have the form of thin ribbons of a thickness less than 4 microns. The film-fibril elements, often found as aggregates, intermittently unite and separate at irregular intervals called tie points in various places throughout the width, length and thickness of the strand to form an integral three-dimensional plexus. The film-fibrils are often rolled or folded about the principal film-fibril axis, giving the appearance of a fibrous material when examined without magnification. The strand comprising a three-dimensional network of film-fibril elements is referred to as a plexifilament. The plexifilaments are unitary or integral in nature, meaning the strands are one piece of polymer, are continuous in nature, and the elements which constitute the strand are cohesively interconnected. Minor physical treatments of the continuous strand such as shaking, washing, or textile processing will not cause appreciable amounts of the film-like elements to separate from the strand. The fibrillated strand which forms has an unusually high surface area per gram, and is useful for a number of purposes where high adsorption is important, e.g., in cigarette filters.

The exceptionally high strength makes it also suitable for various other applications. Thus the fibrillated strand can be beaten into small particles and formed into a synthetic paper. For still other purposes the strand can be collected continuously on a belt or a screen with the batt then being compressed to form a high tenacity sheet ma terial which is useful for a number of applications, such as in making light-weight irrigation pipe, wall covering, or tarpaulins.

In developing a process for spinning a continuous strand by the foregoing flash spinn ng technique, frequently difficulties have been experienced in consistently obtaining strands with continuous uniform morphology throughout their lengths. Particularly this has been so for the production of the class of plexifilamentary strands described in the aforementioned US. patent wherein each fibrillated strand is very fibrous in nature and is an open network of narrow ribbon-like elements or film fibrils generally coextensively aligned with the longitudinal axis of the strand.

It has been observed with such techniques that changes in temperature, pressure, or concentration during spinning frequently cause segments of the strand to vary in morphology along the length of the strand. Thus although much of the strand is composed of the highly fibrillated structure which is desired, certain portions may be poorly fibrillated or foamy. Investigation has shown that these changes in morphology are primarily a result of phase changes which occur during spinning. Thus under certain conditions the spinning solution forms a cloudy dispersion which, if allowed to stand without adequate agitation, settles into two distinct layers, one layer being rich in polymer and the other layer being lean in polymer. If process controls fail to take in account these phase relationships, large differences in morphology can occur along the length of the strands which are flash spun from the solutions.

Accordingly an object of the invention is to provide a process and apparatus for producing elongated shape materials from a polymer solution.

A further object of the present invention is to provide a process and apparatus for spinning a strand with uniform morphology from a solution of an organic polymer in which the solution temperature is near to or greater than the critical temperature of the solvent.

Another object is to provide a highly efficient process and apparatus for dissolving polymer continuously and spinning a fibrillated strand or web without the use of intervening spinning pumps.

Another object is to provide a process and apparatus for continuously preparing a solution and forwarding it continuously to two or more spinnerets and spinning fibrillated strands of comparable morphology from each of the spinnerets.

Another object is to provide a process and apparatus for continuously dissolving an organic polymer and spinning fibrillated strands from a number of spinnerets in parallel, wherein the number of spinning positions in parallel can be increased or decreased without changing the morphology or continuity of the original strands.

These objectives are accomplished by a process and apparatus for continuously supplying polymer and solvent at a fixed ratio into a dissolving zone, forming a solution in the dissolution zone, forwarding the resulting solution through a transfer zone, controlling the pressure in the transfer zone by varying the rate of supply to the dissolution zone, then passing the solution through a constriction to a pressure let-down zone, and finally allowing the solution to escape through a spinneret orifice of limited size such that the solvent evaporates instantaneously and polymeric material of constant morphology precipitates.

over extended periods of operation.

More particularly according to the process of the in vention fibrillated plexifilamentary material is produced by first continuously supplying under pressure into a dissolution zone, synthetic crystallizable organic polymer of filament forming molecular weigh-t and an inert solvent for the polymer, the concentration of polymer being 2 to 20% by weight of the solution. The polymer is dissolved in the zone and a polymer solution is produced having a temperature of at least the solvent critical temperature minus 45 C. and a pressure above the twoliquid-phase pressure for the solution. Thereafter the solution is forwarded through a transfer zone while maintaining a heat balance at substantially the same level as in the dissolution chamber. A constant pressure above the tWo-liquid-phase pressure is maintained in the transfer zone by control means such that the total supply of polymer and solvent to the dissolution zone is varied inversely in relation to the pressure in the transfer zone. At the same time a fixed ratio of polymer and solvent is maintained. Subsequently the solution is passed into a pressure let-down zone for lowering the pressure of the solution to a pressure below the two-liquid-phase pressure for the solution. Finally the dispersion is discharged through a spinneret orifice of restricted size to an area of substantially atmospheric pressure and temperature to yield the fibrillated plexifilamentary material.

As will be described in greater detail hereinafter, the polymer and solvent may be supplied individually to the dissolution zone, e.g. molten polymer from one fiow line and heated solvent from another, or in combination, e.g. as a slurry. In either case the polymer/solvent ratio is maintained substantially fixed or constant.

It has been found that by the process and apparatus of the present invention the morphology, e.g. denier, degree of fibrillation, bulk, etc., of flash-spun strands can be kept remarkably uniform by spinning under conditions which promote uniform spinning pressure. By automatic regulation of the throughput an essentially constant spinning pressure can be maintained such that the quality and uniformity of the product is remarkably consistent While uniform spinning pressures are a desirable characteristic of even conventional spinning techniques, the problem is particularly severe in the case of flash spinning processes for producing fibrillated plexifilaments because of the considerable extent to which the spinning conditions, rather than the orifice size and shape, directly affect the structure of the resultant filament. By the process of the present invention primary control of forward motion for the spinning solution up to the final orifice is provided through the pump or pumps which feed polymer and solvent into the dissolving zone.

The invention will be described by reference to the figures:

FIGURE 1 is a flow diagram showing elements of one embodiment of the dissolving and spinning process of this invention wherein a solution is continuously formed and passed through a preliminary orifice into a pressure let-down zone and then is passed through a final orifice to the surrounding atmosphere.

FIGURE 2 is a drawing of a spinneret having a pressure let-down zone.

FIGURE 3 is a flow diagram showing a continuous dissolving and spinning process in which the solution is formed from separate supplies of polymer and solvent and is delivered through an automatic valve to a pressure let-down zone and then is passed to a single spinneret orifice.

FIGURE 4 is a flow diagram for a system for distributing solution from a transfer line to several spinnerets and controlling pressure let-down in each of the spinnerets.

FIGURE 5 is a graph illustrating the phase changes encountered with polymer solutions at high temperatures and pressures.

4 FIGURE 6 is a flow diagram similar to FIGURE 3 except that a slurry is first formed of polymer and solvent.

One of the forms of the invention is illustrated by the flow diagram of FIGURE 1. Molten polymer is fed by a polymer pump 1 into continuous dissolving zone 2, and heated solvent is pumped by solvent pump 3 through solvent heater 7 to the dissolving zone. In this way thorough contact of the low viscosity solvent with the high viscosity polymer is assured upon blending. A solution is formed continuously in the dissolving zone, the weight ratio of polymer to solvent being controlled by the relative pumping rates of the polymer and solvent.

The solution which is formed in the dissolving zone is forced continuously from the dissolving zone into a transfer zone 4 which comprises a part of the flow line extending from the dissolution zone to the spinning orifice.

The solution passes continuously from the transfer zone to a pressure let-down zone which constitutes one element of what will be termed a spinneret assembly, indicated in the figure as 5. The latter also comprises a spinning orifice 6 having dimensions small enough to prevent the full capacity of the polymer and the solvent pumps from being exceeded. The solution is extruded from the spinning orifice to the surrounding atmosphere, normally at ambient temperature and pressure, at a very high speed whereupon the solvent evaporates at an extremely rapid rate, forming continuously a fibrillated strand 8 with very constant morphology. The solution in the transfer zone is maintained at a temperature level which is equal to or higher than the critical temperature of the solvent minus 45 C. The temperature of the solution in the transfer zone is controlled by a temperature sensor unit 9 in the transfer zone which feeds a signal to tempertaure control unit 10, which in turn regulates the amount of heat applied by the solvent heater element 7. Heat losses throughout the system are desirably balanced by means of electric heating elements or fluid jackets surrounding the various process units.

The solution in the transfer zone is maintained at a specified pressure which is higher than the two-liquidphase pressure of the solvent and is maintained within :50 p.s.i., preferably within :15 p.s.i., of the specified pressure by means of a pressure control unit 11. Owing to the oscillation of actual pressure as it deviates from the selected mean pressure, care must be exercised to ensure that the pressure level be set sulficiently high in the transfer zone to avoid the possibility of pressure occasionally falling below the two-liquid-phase pressure boundary during continued periods of operation. The pressure control unit is activated by a pressure sensor element 12 in the transfer zone. The pressure sensor feeds a signal to the pressure control unit which in turn feeds a signal to the speed level and speed ratio control unit 13. This control unit 13 controls pressure by controlling the overall speeds of the polymer pump 1 and the solvent pump 3 without changing the speed ratio between the two pumps. The maintenance of a constant spinning pressure with a fixed pump speed ratio gives an essentially uniform flow rate.

Normally it is desirable to achieve a total pressure drop which is sufiicient to ensure the continuous formation of the aforementioned phases irrespective of the slight pressure deviations from the selected mean pressures. A number of the forms of the invention are of course possible. The pressure let-down, one in particular can vary somewhat in structure provided a pressure drop is realized from above the two-liquid-phase pressure to below it. For example the pressure let-down zone may consist simply of a small chamber in the spinneret assembly having an inlet constriction and an outlet constriction defining a let-down zone of a volume as low as 0.5 cc. or less. The throughput rate must be suiticiently high to prevent actual separation of two polymer solution phases which must be formed in the let-down zone, i.e. downstream of the first constriction. The drop in pressure, desirably to at least 5 p.s.i. below the twoliquid-phase pressure, should not be so great as to actually cause appreciable precipitation of polymer or vaporization of solvent. Preferably the pressure in the letdown zone is kept above the autogenous pressure of the solvent at its critical temperature minus 45 C.

In some instances it is convenient to simply employ a conduit such as a pipe line as a let-down chamber. In one embodiment, which is further described hereinafter, the solution passes from the transfer zone through a constriction formed by an automatic valve, then to the pressure let-down zone defined thereby and finally through a single orifice to the surrounding atmosphere. In this case there are two successive pressure control loops, one having a sensor unit and control means in the transfer zone, and the other having a sensor unit and control means in the pressure let-down zone.

In polymer-solvent systems where two liquid phases form readily, the drop in pressure in the let-down zone causes two liquid phases to form, with tiny droplets of one hase being carried in the second phase. The tiny droplets apparently act as bubble nuclei and promote an extremely high degree of fibrillation in the threadline when it emerges into the surrounding air at atmospheric pressure. Maximum fibrillation is of particular importance for achieving the greatest degree of opacity, bulk and other desirable fiber properties. Arrangement of the pressures in the system so that the pressure in the flow line upstream of the constriction is above the twophase pressure limit, assures the attainment of such high degrees of fibrillation. As an added safeguard for a commercial installation employing typical available high pressure equipment, a pressure drop from at least 25 p.s.i. above the two-liquid phase pressure to at least 25 psi. below that pressure is desirably provided.

Referring to FIGURE 3, a flow diagram is shown for a system having two pressure control loops. The first control loop has a sensor unit in the flow line between the dissolution zone and the constriction and controls pressure by controlling the speed of the polymer and solvent pumps. The second control loop is separated from the first by the constriction which is an automatic valve whose capacity is such that it operates between 20 and 80% open at all times. In the second loop, pressure is controlled by a means of sensor unit in the pressure let-down zone which is down-stream from the automatic valve. The latter pressure sensor unit feeds a signal to a controller which in turn regulates the amount of valve opening.

The provision of the second control loop, while not essential to the invention, greatly contributes to the overall flexibility and efiiciency of the system. For example, by virtue thereof it becomes practical to employ a series of spinnerets, each supplied by a common flow line, which can be so adjusted as to give strands of similar or dissimilar morphology. This feature will be later described in greater detail with reference to Example III and FIGURE 4.

In FIGURE 3 polymer cubes are supplied to the polymer hopper 41 and thence fed to an extruder 42 which is driven by electric motor 43. The polymer is melted and pressurized by this feed device being then screw pumped through the polymer transfer line 44 to a dissolution zone, comprising screw mixer unit 45 and variable speed electric motor drive 46, where it is initially mixed with solvent. A second feed device is provided for introducing hot solvent under pressure to the dissolution zone. Solvent is supplied from solvent storage tank 72 which is desirably maintained under 15 p.s.i. nitrogen pressure to prevent vapor locking of solvent supply pump 71. The solvent supply pump 71 transports solvent through the low pressure solvent line 64 to the solvent preheater element 63. The solvent preheater element warms the solvent to a temperature slightly in excess of the maximum ambient solvent temperature experienced in year-round operation and therefore is a convenient means for insuring that a constant density solvent is fed to the volumetric solvent pump 61 The solvent pump, driven by electric motor 62, umps high pressure solvent through the cold, high pressure solvent line 60 to the solvent heater element 5% which heats the solvent to the desired process temperature. From the solvent heater element 58 solvent passes through the hot, high pressure solvent line 59 to the screw mixer unit 45 where it is initially mixed with polymer and where a solution is formed. Formation of a well blended homogeneous solution of polymer is, of course, essential to the consistent production of strands of uniform morphology. Ordinarily a screw mixer unit of suficiently large capacity in relation to the desired throughput will readily achieve this result. For a large scale commercial operation, however, it is often desirable to provide following the screw mixer a second agitation device to fully ensure dissolution of the polymer and to readily smooth out any short term concentration fluctuations. Thus FIGURE 3 indicates that from the screw mixer unit the solution passes through the solution transfer line 47 to the high shear zone 48 of a typical averaging mixer unit 49 and then to the main body of the averaging mixer unit. The capacity of the averaging mixer unit, driven by an electric motor 73, can thus be sufficiently large to provide thorough agitation of the solution.

From the averaging mixer unit the solution passes through the solution transfer line 50 to the dual solution filter assembly 54 to remove any foreign particles or undissolved polymer. Following the filters the solution passes to transfer line 85 having a pressure sensor such as an electronic Swartwout pressure transmitter 51 which serves to determine, record, and signal the pressure in the transfer zone.

Pressure in the transfer zone, i.e., that portion of the flow line extending from the dissolution zone to the constriction, is regulated and controlled by pressure control means which vary in a fixed ratio the speeds of the extruder 42 and the solvent pump 61 inversely in relation to the pressure in the transfer zone. In a start-up of the operation, for example, a spinning pressure is selected from a consideration of the solvent, polymer, throughput and desired degree of fibrillation of the product. A temperature is selected above the critical temperature, Tc, of the solvent minus 45 C. For sake of convenience establishment of an initial pressure slightly above the twoliquid-phase pressure of the solvent at the selected temperature will be satisfactory. Further discussion of this aspect is given hereinafter with reference to Table I. The set point of the pressure control unit 68 is then adjusted to the selected pressure. In operation the system pressure is sensed by pressure transmitter 51 which is then immediately transmitted to the pressure control means. The latter comprises units 68 which senses any deviation of the system pressure from the selected pressure and feeds an appropriate correction signal to the speed level and speed ratio control unit 67 thereof. That unit then directs changes in the drive speeds of drive motors 62 and 43 for the solvent pump and polymer extruder, as required to offset the deviation and return the pressure to that selected by the set point of pressure control unit 68. When, for example, the pressure in line decreases as a result of partial blocking upstream, pressure control unit 68 upon being fed the pressure reading by transmitter 51 signals the deviation to control unit 67 which directs an increase in the speed of extruder 42 and solvent pump 61. Should the partial blocking eventually be removed, control unit 67 would direct a decrease in the flow rate until the selected pressure was once again obtained.

The speed level and speed ratio controller 67 of the control means not only regulates the speeds of the drive motors but also maintains a predetermined ratio of speeds between the extruder 42 and the solvent pump 61. In

order to accurately control the speeds and speed ratio of the drives 43, 62, they are equipped with tachometer devices 65, 66 which supply a feed-back signal to the drive speed control unit 67.

The control of concentration is effected by controlling the ratio of the speeds of the extruder device 42 and the solvent pump 61. This control is efiected by the drive speed control unit 67. The speed ratio is determined from calibrations of the solvent pump 61 and the extruder device 42. The solvent pump 61 is a volumetric metering device. For this reason a solvent preheater element 63 is installated to heat the solvent to constant temperature to eliminate seasonal and diurnal changes in density which would be caused by temperature fluctuations.

The temperature of the solution in the transfer zone is controlled at substantially the same level as in the dissolving zone by means of a temperature sensor element 83 on transfer line 47, which sends a signal to temperature control unit 84, which in turn regulates the steam temperature in the jacket of solvent heater element 58 inversely in relation to the sensed temperature.

A pertinent consideration in the design of the screw extruder 42 is that it must be capable of generating and maintaining high pressure. Conventional extruders available commercially are entirely satisfactory for use in the high pressure system of this invention provided that the clearance between the screw and interior surface of the barrel is small, the discharge orifice is small in relation to the barrel diameter, and provided that adequate power is available to ensure continuous rotation.

A screw mixer which is particularly satisfactory for use as the dissolving zone 45 in the above description is described in U.S. Patent 3,006,029. For purposes of this invention an inlet pipe for solvent can be provided at a point along the length of the barrel.

From the solution transfer line 80 there is a branch line 81 having a ballast valve 53 which may be opened during temporary shutdown of the spinning zone to keep solution flowing through continuous dissolving and transfer zones. In this way gelatin or decomposition which might otherwise occur can be avoided.

The system is equipped with dual filters 54 so that operation can be maintained during a filter change. The filters can be composed of fine wire screens or other materials.

From the dual filters the solution passes through a transfer line 80, passes a pressure transmitter device 51, and then passes a constriction comprising an automatic let-down valve device 55 that reduces the pressure to a level set by the pressure let-down control unit 70. The latter is a component of a second pressure control means which serves to vary flow through the valve inversely dependent on the pressure downstream of the constriction. The amount of pressure drop is adjusted to give the degree of fibrillation desired in the product. The pressure transmitter and sensor element 69 which supplies the signal to the spinneret pressure control unit 70 is located in transfer line 82 downstream of the automatic let-down valve 55 and upstream of the spinneret 92 and final orifice 56. Once the necessary pressure let-down is established to afford the desired degree of fibrillation, the position of the valve is essentially fixed for normal operations, since in the absence of a complete obstruction of the transfer lines upstream the pressure immediately upstream of the valve is constant. For purposes of establishing the desired degree of pressure let-down it will be apparent that the smaller the opening in the valve the higher will be the pressure drop and vice-versa.

The spinning orifice dimensions which may be used with the apparatus of FIGURE 3, depend upon the solution viscosity, temperature, desired throughput rate, and upon the pressure just upstream of the final orifice. The data of Table I can be used to determine the orifice crosssectional area for a given set of pressure and throughput rates for a 14% solution of linear polyethylene of melt 8 index .56 in trichlofiuoromethane when the transfer zone temperature is 185 C. In general the distance through the orifice passage (length) is kept the same as the orifice diameter.

TABLE I 0.015 inch 0.025 inch 0.032 inch In one embodiment of the invention a flash spinning apparatus having a pressure let-down chamber built into the spinneret assembly can be used in place of the automatic valve device 55, the let-down pressure sensor element 69, the pressure control 70 and the single orifice spinneret 56 of FIGURE 3. In such an arrangement the pressure let-down spinneret is attached directly to transfor line following pressure sensor element 51. The design of this spinneret is shown in FIGURE 2. The solution enters the spinneret through the constriction 22 and passes to the let-down chamber 23 and finally is extruded through spinning orifice 24 into the surrounding atmosphere. This spinneret of course must be designed for a specific throughput rate, and for specific transfer zone pressures. At a given throughput rate and transfer zone pressure, it will provide a constant pressure let-down. The system in which the automatic valve is used instead of the let-down spinneret is preferred when the operator wishes to retain freedom to change operating conditions to meet demands for various types of strand morphology without changing spinnerets. It is also useful when a plurality of strands is to be produced from several spinnerets having a common transfer line in that the need for several operators is eliminated.

The system of the invention will readily compensate for any decrease in pressure in the transfer zone by directing an increase in throughput of both solvent and polymer until the selected pressure becomes once again established. Frequently such decreases in pressure will arise as polymer gel particles lodge within the transfer zone and partially obstruct the passage. Since excessively high pressures may occasionally be experienced upstream of an obstruction it is normally desirable to provide in the transfer zone a blow-off valve or other shut-off means, not shown, to ensure that the pressure capacity of the system will not be exceeded.

While the pressure control means for the transfer zone would not effectively counteract any variations in pressure due to obstructions between the pressure let-down zone and the orifice, in practice these have been found to be rare and indeed of little consequence.

For some purposes it may be desirable to employ both the automatic valve system and the pressure let-down spinneret. In this case there are actually two constrictions in the flow of solution prior to strand formation: (1) at the valve, and (2) at the entry orifice of the spinneret. The final orifice then constitutes a third constriction. When the automatic valve and pressure let-down spinneret are both employed, the pressure sensor element may be either located in the flow line preceding the spinneret or may lie in the spinneret let-down chamber.

In another embodiment of the invention more fully described in Example III hereinafter with reference to FIGURE 4, several strands may be spun from the same continuous solution supply using a manifold to distribute the solution to a number of spinnerets. Automatic valves followed by pressure sensor elements can be located in each branch of the manifold to control morphology of the strand from each spinneret individually. By this means it is entirely practical to produce strands of differing morphology from the same solution supply by establishing different let-down pressures in various spinnerets.

A wide variety of components may be used in the apparatus so long as they fulfill the prescribed functions. In one preferred system the polymer pump is a screw extruder which melt the polymer largely by means of the heat generated in mechanically working the polymer. Also the transfer zone may take a variety of forms. It may be a simple length of pipe capable of withstanding super critical temperatures and super autogenous pressures, or it may include a stirred autoclave or other device for averaging out the solution concentration and for assuring complete solution of the polymer.

A second, also preferred, form of the invention is illustrated by the flow diagram of FIGURE 6. Powdered polymer obtained from a grinder, an impact mill, a hammer mill or the like is fed through hopper 101 to polymer storage tank 102. A supply of solvent is obtained from solvent storage tank 103. The powdered polymer and solvent are led through low pressure lines 105 and 105, respectively, into cold slurry tank 109 which is provided with a propeller 111 powered by drive motor 110. The concentration of powdered polymer in the solvent carrier i adjusted to the exact ratio desired by means of standard metering devices, not shown, in lines 105 and 106. For low boiling solvents the low pressure slurry tank 109 may require cooling coils, also not shown, to eliminate any solvent losses. Cold low pressure slurry in tank 109 is fed via line 112 to cold slurry pump 115. A suitable pump for this purpose is De Lavals Hydropulse pump model H-3 which is a double diaphragm positive displacement pump with ball check valves. The pump produces a cold slurry at a pressure well above the two-liquid boundary pressure for the solution which will eventually be produced. The cold slurry pump advances the slurry through line 120 into slurry heater 121. Turbulent fiow in line 120 and heater 121 prevents separation of the polymer from the solvent carrier. From slurry heater 121 the heated mixture passes through line 122 into mixer 123, driven by motor 124, to complete dissolution of the polymer. The mixer is provided with four propellers 125 mounted on a single shaft and includes baffles 127 separating the mixer into four stages. The slurry heater 121 and mixer 123 together comprise the dissolution system. Although a sufliciently high temperature is achieved in the heater 121, much of the polymer does not actually dissolve until it reaches the mixer 123.

Remaining elements of the two-pressure control loop apparatus of FIG. 6 are similar to those described with reference to FIG. 3 above. The pressure control means varies the output of the cold slurry pump 115 inversely in relation to the pressure in the transfer zone. Thus pressure transmitter 51 senses the system pressure and transmits same to the pressure control unit 68. The latter through level control transducer 107 directs changes in the throughput of pump 115. The temperature of the solution in the transfer zone is controlled at a substantially constant level by means of pressure sensor element 33 on transfer line 47 which sends a signal to temperature control unit 84. The latter in turn regulates the temperature of slurry heater element 121 inversely in relation to the sensed temperature. Sensor element 83 can be physically located anywhere in the apparatus downstream of heater 121.

In providing a fixed pressure drop and constant spinning pressure to ensure uniform product morphology, the invention affords still other significant benefits. Thus it becomes practical to provide a filtration system to remove from the stream of polymer solution undesirable foreign particles, polymer specks, gel particles, etc., without concern that the gradual buildup of such particles on the filters will deleteriously affect the pressure in the flow line. Specifically, any tendency of the solution pressure downstream of the filter to diminish as such buildup ensues is readily overcome by the pressure sensor and the control units which would direct an increase in pump speeds to ensure delivery at pressures above the two-liquid-phase pressure.

The highly fibrillated plexifilamentary strands produced in accordance with the improved process and apparatus of this invention are described in detail in aforementioned US. Patent 3,081,519, the disclosure of which is specifically incorporated herein by reference.

The whole strands can have deniers as low as 15 or as high as 100,000 or even higher. Preferably, they have deniers between 100 and 10,000. For purposes of this invention they should exist in a highly fibrillated form which has the appearance of sliver or tow of extremely fine fibers. The film-fibrils, however, are connected in a network, there being few if any unconnected fibril ends. The strands further have tenacities after twisting 8 turns per inch of at least 1.0 g.p.d. and when drawn give tenacities as high as 23.0 g.p.d. All of the strands are characterized morphologically by a three-dimensional network of film-fibril elements. These networks may exist in various forms, but in all cases the film-fibrils are extremely thin. On the average the film-fibril thickness is less than 4 microns thick. In the preferred products the film-fibrils are less than two microns thick and may indeed have a thickness of less than 1 micron. The film-fibril elements are at least five times as wide as they are thick, the actual width being between about 1 micron and about 1,000 microns. The thickness of the film-fibril elements may be determined by use of the interferometer microscope.

The film-fibril elements in plexifilaments are found in the form of fibril composites which are laminates, aggregates or bundles within the gross strand. Because these fibril composites continuously divide and parts of them join other bundles, it is difiicult to count individual film-fibrils in the strand. However, for convenience, the average number of fibril composites in a 0.1 mm. thick cross-sectional cut of the strand is used as a measure of degree of fibrillation. The number of these fibril composites per 1,000 denier in a 0.1 mm. length of strand is referred to as the free fibril count. It is recognized that the number of additional film-fibrils which can be pulled away from the fibril composites with slight tension will be many times the number found already free, but film-fibrils which adhere to each other are not counted as separate fibrils in the standard test.

The free fibril count is determined by freezing and sectioning techniques followed by microscopic examination as described in the above mentioned United States Patent 3,081,519. The data are reproted as free fibrils/1,000 denier/0.1 mm. length. The free fibril count for the fibrillated species is at least 50 free fibrils/1,000 denier/0.1 mm. length and counts of 1,000 or higher are often obtained with strands of this species. These fabrillated strands will however, have a minimum of 25 free fibrils per 0.1 mm. length regardless of denier.

Preferably more than half of the fibrils in a strand have lengths under 1.5 cm. (i.e., between points of attachment). The tie points being spatially arranged in vari ous planes along the width, length and depth of the strand are responsible for the three-dimensional structure which results.

The predominantly longitudinal orientation of the filmfibrils of all plexifilament strands is readily apparent from the fact that all such strands are much more resistant to tearing or breaking transversely than to splitting lenghwise. The general coextensive alignment of the fibrous elements in the direction parallel to the strand axis is easily discernible to the naked eye for most plexifilamentary species.

The plexifilamentary strands of the invention are made of crystalline polymer. The pellicular material in the as-spun strand when consisting of a crystalline polymer is substantially oriented as measured by electron diffraction, i.e., it has electron diffraction orientation angles smaller than It is believed that the high strength of the plexifilamentary strand as spun is closely related to the crystalline orientation within the film-like ribbon and in the structural arrangement of the fibrils themselves in the strand. In the preferred crystalline oriented products, the film fibrils have electron diffraction angles of less than 55. The orientation of the crystallites in the filmfibrils is in the general direction of the film-fibril axis.

X-ray difiraction patterns which are obtained using the whole strand instead of just film-fibrils show a substantial amount of orientation in the strand as spun. The X-ray ditfraction orientation angles are less than 55 in the preferred embodiments of the invention. The substantial orientation which is exhibited by the gross strands indicate that not only are crystallites oriented along the fibrils, but the fibrils are themselves oriented in the general direction of the strand.

Plexifilament strands have a surface area greater than 2 m. /g., as measured by nitrogen absorption methods. Due to the extremely high polymer/air interfacial area the strands have marked light scattering ability and high covering power. Still another important characteristic of the strands is the fibrillar texture of the gross strand as observed with the polarizing microscope.

The strands have a high. degree of organization, and the highly organized areas extend for considerable distances along the length of the strand. The strands are characterized as fibrillar if at least half of the material making up the strand appears as monochromatic streaks when observed in the polarizing microscope. The monochromatic streaks are oriented in the direction of the strand axis and have an actual (unmagnified) length of at least 0.2 mm.. The monochromatic areas are considered as streaks when they have a length at least 10 times the Width.

The fibrillated plexifilament is a soft, supple strand having the outward appearance of a bulky, staple spun yarn. When examined at 400X magnification, the film fibrils have the appearance of ribbons of extremely thin pellicular material, folded or rolled approximately about the film-fibril axis. For this reason they appear to be fibrous when examined without magnification.

In the most preferred form, the fibrillated strands can be spread transversely to many times their original width without breaking any appreciable number of film-fibril elements. In general, the film-fibrils separate instead of breaking when the strand is stretched transversely.

The plexifilaments are prepared from synthetic filamentforming polymers or polymer mixtures which are capable of having appreciable crystallinity and a high rate of crystallization. Necessarily the polymer must be of such a nature that a solution formed thereof in accordance with the invention can undergo the requisite two-liquid phase phenomenon within the pressure let-down zone. A preferred class of polymers is the crystalline, non-polar group consisting mainly of crystalline polyhydrocarbons. Examples of these include linear and branched chain polyethylene, polypropylene, copolymers of ethylene with other olefins, etc. Other crystalline polymers such as polyethylene terephthalate, copolymers of ethylene with other monomers can also be employed. Common textile additives such as dyes, pigments, antioxidant-s, delusterants, antistatic agents, reinforcing particles, adhesion promoters, removable particles, ion exchange materials, and U.V. stabilizers may be mixed with the polymer solution prior to extrusion.

Suitable liquids for use in forming the high temperature, high pressure polymer solutions required for forming the plexifilaments should preferably have the following characteristics: (a) a boiling point (atmospheric pressure) at least 25 C. below the melting point of the polymer used; (b) it should be substantially unreactive with the polymer during extrusion; (c) it should be a solvent for the polymer under the temperature and pressure conditions suitable in this invention as set forth below; ((1) it should dissolve less than 1% of the high polymeric material at or below its normal boiling point; and (e) the liquid should form a solution which will undergo rapid phase separation (i.e., in less than .01 second) upon extrusion forming a non-gel polymer phase, i.e., a polymer phase containing insufiicient residual solvent to plasticize the structure. In these requirements, the process differs radically from conventional solution spinning techniques, wherein the spinning solvent is invariably a solvent for the polymer below the normal boiling point and generally is a solvent at room temperatures.

Among those liquids which may be utilized in the spinning process, depending upon the particular polymer used, are aromatic hydrocarbons such as benzene, toluene, etc.; aliphatic hydrocarbons such as butane, pentane, hexane, heptane, octane, and their isomers and homologs; alicyclic hydrocarbons such as cyclohexane; unsaturated hydrocarbons; halogenated hydrocarbons such as methylene chloride, carbon tetrachloride, chloroform, ethyl chloride, methyl chloride; alcohols; esters; ethers; ketones; nitriles; amides; fiuorocarbons; sulfur dioxide; carbon disulfide; nitromethane; water; and mixtures of the above liquids.

Among the wide variety of polymers and solvents which can be used in accordance with the invention, particularly suitable combinations are illustrated by linear polyethylene (density of 0.940.98 g./cc.) with either cyclohexane, methylene chloride, trichlorofluoromethane, pentane or butane. Various combinations of those or other solvents with one or more polymers can often be used to advantage and it will be apparent that such selection is within the skill of the art for producing a product of the desired character.

Aforementioned United States Patent 3,081,519 diagrammatically illustrates certain principles helpful in establishing optimum spinning conditions and these are also appropriate to the instant invention. As indicated therein, fibrillated structures are obtained at temperatures above Tc45 C. and may be obtained at temperatures even above the solvent critical temperature. It is preferred to use conditions of temperature above the normal melting point of the 100% pure polymer, and polymer concentrations of between about 2% and 20%, weight basis on the solution. Thus a linear polyethylene-methylene chloride system yields a fibrillated product when the polymer concentration on a weight percent basis is between 2 and 20% and the temperature is above (T c45 C.) or 193 C.

It is preferred to operate the extrusion process at velocities which produce more than about 3,000 yard-s of plexifilament per minute. At these velocities, an internal orientation force is exerted on the polymer solution during the brief formative interval in which the polymer solution undergoes transition first to a system containing vapor bubbles and polymer solution, and thence to a shaped solid; during which time critical transient viscosity and velocity gradients exist. The orientative force facilitates the general longitudinal orientation of the fibrils which characterizes plexifilaments. Desirably the dwell time of solution in the pressure let-down zone is at least 0.06 second but not more than 30 seconds.

Plexifilamentary strands can be produced at velocities as high as about 17,000 y.p.m. and higher. The extrusion velocity appears to be generally dependent upon the pressure gradient across the orifice, the orifice length and cross-sectional area, solution viscosity, and the geometry of the low pressure side of the orifice. The pressure within the extrusion vessel may be increased by using higher temperatures to obtain higher pressures or by using mechanical pumps.

The design of the orifice and neighboring structural elements aflfects the nature of the product obtained. For example, the spinneret device of FIGURE 2 provides a chamber 23 having an intermediate pressure between the extrusion orifice 22 of the vessel and the actual spinning orifice 24, thereby producing nuclei for eventually inducring bubble formation within the spinning orifice. Nucleation gives rise to a high bubble count with subsequent production of highly fibrillated structures of relatively low strand deniers. Other spinneret and orifice designs may be employed such as multihole spinnerets, spray jets wherein the immediately extruded stream impinges on a surface which redistributes the stream, swirl jets, slot orifices, annular orifices, and the like.

The orifice cross section may be of any simple shape but it is preferred that the smallest cross-wise dimension be at least about 4 mils.

In promoting maximum uniformity and maximum fibrillation during spinning it is essential to prevent formation of two phases until the solution has reached the pressure let-down zone, otherwise the two phases will stratify into layers or into large droplets and will cause thin spots and lumps to form in the strand or will give other undesirable variations in morphology along the line.

The two-'liquid-phase pressure boundary will be described with particular reference to FIGURE 5, which is a phase diagram for solutions of linear polyethylene in trichlorofiuoromethane. The ordinate for the graph is temperature, C., the abscissa is absolute pressure, p.s.i.a. Gauge pressures are 1-5 p.s.i. less than p.s.i.a. values. Line A of the graph gives the vapor pressure of the solvent trichlorofluoromethane at various temperatures. Dotted line C shows the temperature limit T c45 C. below which only poorly fibrillated products are obtained. Dotted line D shows the critical temperature of the solvent, while dotted line E shows the critical pressure.

The two-liquid-phase pressure boundary for the 14% solution of linear polyethylene (melt index 0.57) in trichlorofiuoromethane at various temperatures is represented by line PG. The system at temperature-pressure combinations above line FG consists of two liquid phases, a polymer rich liquid, and a polymer lean liquid. On the other hand with temperature-pressure combinations below line FG, the system consists of a single liquid phase. The dotted lines parallel to PG represent other boundary conditions for other concentrations as described in subsequent paragraphs. Thus in operation of the invention with a 14% solution of linear polyethylene (melt index 0.57) in trichlorofiuoromethane, the solution up-stream of the first constriction will have a temperature-pressure relationship within the area below line FG. For example, the solution up-stream of the first constriction might be maintained at a temperature of 187 C. and a pressure of 1600 psi. as indicated by point Y on the graph. Under these conditions the solution in the pipe line of the transfer zone would consist of a single liquid phase. When such a solution passes through the first constriction into a pressure let-down zone, its pressure drops considerably, for example to 860 p.s.i., while the temperature drops only a few degrees to 185 C. as represented by point Z on the map. Under instantaneous conditions at point Z the solution would consist of two liquid phases in the form of a dispersion. The continuous phase would consist of a solution of linear polyethylene of relatively high con centration compared to the dispersed phase. The dispersed phase would be essentially pure solvent with a very small amount of linear polyethylene dissolved in the solvent. The residence time within the let-down chamber must be kept sufliciently brief to prevent separation of the two phases into distinct layers. In the absence of dispersion stabilizing treatments, e.g. stirring, such residence time is preferably kept below 30 seconds. If thedispersion at point Z is retained only momentarily in the let-down chamber, it passes from the final constric tion, i.e. the spinneret orifice, into the atmosphere in a very finely divided dispersed form, and the solvent evaporates instantaneously giving a highly fibrillated strand of linear polyethylene. If the residence time in the letdown zone substantially exceeds 30 seconds the two phases are likely to separate into layers or into large droplets. Commonly a strand produced under such conditions would be discontinuous or would otherwise possess non-uniform morphology.

As will be apparent from FIGURE 5, a wide variety of conditions can be used to obtain the desired highly fibrillated strand. By experimentation it has been found that the location of line PG shifts upward and to the left when solutions of higher concentration are used and shifts downward and to the right for lower concentrations. In FIGURE 5 the lines F 6 F 6 PG, and F 6 rep resent boundary conditions for solution concentrations of 10%, 12%, 14%, and 16%, respectively, for the linear polyethylene/trichlorofluoromethane system, using linear polyethylene With a melt index of 0.57 gram/l0 min. The various boundary lines are substantially parallel.

The location of the two-liquid phase pressure boundary for a given temperature can be altered by changes in melt index (melt index being inversely related to molecular Weight). While the lines on the figure are drawn for a polymer having a melt index of 0.57, a set of nearly parallel lines can be developed for polymer having diiterent melt indices. It has been found that an increase in melt index causes the boundary line to shift downward and to the right. A decrease in melt index causes boundary line to shift upward and to the left.

Similarly the introduction of a relatively insoluble gas such as nitrogen causes the boundary line to shift dramatically downward and to the right. Although introduction of such a gas increases the degree of fibrillation, there are certain practical difficulties which ensue when a solvent with very high vapor pressure is used. Since the dissolved gas causes the two-liquid-phase pressure boundary to move to higher pressures, it becomes difiicult and expensive to design equipment which will hold the extreme pressures required in such systems. When using the process of the present invention with solvents of high vapor pressure it is therefore better to avoid the use of insoluble gases as nucleating agents. It is generally preferable to use mechanical means such as pistons or screw extruders for building up pressure. Furthermore, it is important that air be excluded from the system. Desirably, therefore, the solutions should consist only of solvent and plexifilament-forming polymer such that all other gases, liquids and solids are essentially excluded.

Since the spinnable concentrations for flash-spinning are usually well above the level of 2% polymer in solution, the two-liquid-phase pressure for concentrations below that level :are of little practical concern. Actually another phase boundary will be found at very low concentration, usually below 2%, in which the phase relationships are reversed from those discussed in the preceding paragraphs. Thus in the low concentration area the dispersed phase will consist of a small percent polymer in solution while the continuous phase will consist mainly of clear solvent. Solutions or dispersions having low polymer concentration do not give continuous fibrillated strands of uniform morphology and hence are unsuitable in the process of the invention.

The location of the two-liquid-phase pressure boundary may be established for a given polymer and solvent combination by observing the solution at various temperatures and pressures through a high pressure sight glass in an apparatus equipped with a mechanical pump or other means for providing the necessary super-autogenous pressures. The coupled continuous dissolving and flash-spinning apparatus may be easily modified to obtain the necessary data. For example a thick-walled glass tube may be built into the apparatus of Example I shown in FIG- URE 3. The glass tube with pressure fittings is made part of the flow line 82 down-stream from the let-down pressure sensor 69 and upstream of the spinneret 92.

Pressure is applied by means of the screw extruder and solvent pump with the same control system as in Example I. In order to obtain the boundary data the pressure controller 70 and the automatic valve 55 are deactivated. The valve 55 is manually set wide open (enough to prevent any appreciable pressure drop from occurring in the liquid as it passes valve). Then the flow line 82 has essentially the same pressure as flow line 80, this pressure being controlled by pressure controller 68 which is activated by pressure sensor 51.

The experimental data are obtained then for a polymer of known melt index in a solution of known concentration by controlling temperature and pressures in the system at various levels and observing the solution in the sight glass. At pressures above the two-liquid-phase pressure boundary the solution will be clear; at pressures below the two-liquid-phase pressure boundary the solution will be cloudy. \Vhen the data have been collected for a number of temperatures, a graph may be constructed by plotting the boundary pressure for each temperature as in line FG of FIGURE 5. It is desirable to observe the solutions under static conditions as well as under flow. In this case a needle valve can be used in place of the spinneret 92, and this valve may be closed while the solution in the glass pressure tube is being observed. By controlling the fiow rate through the spinneret, further data can be obtained to establish the optimum residence time for the solution in the pressure let-down zone. If the residence time is too long the solution in the tube will form large droplets and eventually will separate into two layers when the solution is kept at temperature-pressure values below the two-liquid-phase boundary pressure.

When the fibrillated plexifilamentary strand of the present invention is allowed to impinge against a baffle as it flashes from the spinnerts a wide web is formed. The web, which can be several times wider than the issuing strand, contains the film-fibrils and tie points as before in a three-dimensional plexus. Such a web can readily be collected on a moving belt, followed by light compression if desired to form a non-woven sheet material. Also the plexifilaments can be converted on a regular paper machine into a slurry which will form satisfactory papery products for printing, wrapping and related purposes.

The following examples illustrate specific embodiments of the invention. All parts and percentages are by weight unless otherwise indicated.

In the following examples the pressures which are indicated may be either autogenous pressures, i.e., pressures generated by the solvent, or may be higher because of mechanical pressure exerted by the feed devices. The melt index of the polymer, in grams/'10 minutes, is determined by ASTM method D1238-57T, condition E and is inversely related to molecular weight. By linear polyethylene is meant polyethylene having a density of 0.94 to 0.98 g./cm. but preferably having a density of 0.95 to 0.97 g./cm. The polymers are of at least film-forming molecular weight.

EXAMPLE I A single continuous strand of highly fibrillated linear polyethylene is spun using the apparatus and process shown in FIGURE 3. There are two pressure control loops. The first one maintains pressure in the transfer zone upstream of the let-down zone. The second controls pressure in the let-down zone following an automatic valve constriction. The spinneret assembly has a constricion at the final orifice only. The spinneret orifice has a diameter of 0.025 inch and a passage length of 0.025 inch. The let-down pressure sensor is in the flow line between the automatic valve and the spinneret.

Various components of the equipment in the flow diagram of FIGURE 3 will now be described in detail.

The screw extruder which acts as a molten polymer feed device consists of a helical screw operating in a cylindrical barrel. The screw has the following characteristics:

Outside diameter inch 3.5 Flighted length -do 68.5 Number of channels 19 Channel helix angle deg 45 Holdup volume inch Channel depth inch '/2 to A;

The extruder is jacketed in three sections, each equipped with provisions for cooling or heating. For the present example the temperature of the liquid jacket is maintained at 175, 205 and 205 C. respectively from feed to exit by a controller operating from thermocouples in the individual jackets. Most of the heat for melting the polymer is supplied by the mechanical energy of the extruder.

The solvent feed device includes a reciprocating pump having three pistons with a stroke of 2 inches and a diameter of 0.5 inch. The hold-up volume is 1.5 cubic inches.

The molten polymer passes from the extruder into an electrically heated polymer transfer line, the heat loss being balanced by electrical heaters to maintain the molten condition. The molten polymer then passes into a screw mixer which serves as a dissolution chamber. The screw mixer is that described in U.S. 3,006,029, where mixing is effected by means of a helical screw operating in a cylindrical barrel. The helical flights of the screw are interrupted by major helical grooves of the opposite hand. For the purposes of the present invention the extruder has been fitted with an inlet pipe for introduction of the hot solvent near the middle of the barrel length. The mixing screw has the following characteristics:

Outside diameter "inch" 4 Flighted length do 60 Number of channels:

Minor 6 Major 1 Channel depth, in.:

Minor Major A; Channel helix angle:

Minor 1940 Major deg 60 Hold-up Volume inch 440 Heat losses from the barrel of the screw mixer are balanced by an electric heating blanket.

In operating the system, linear polyethylene of melt index 0.5 having a desnsity of 0.953 g./cm. is fed in cube form /s inch) into the hopper of the extruder at the rate of about 24 lbs/hr. The polymer melts in the extruder and exits at a temperature of 203 C. and a pressure of 1900 p.s.i.g. It passes through a heat-balanced transfer line to the screw mixer.

Solvent for the system is supplied from a pressure storage tank. The solvent is trichlorofluoromethane which boils at 24 C., whose critical temperature, Tc, is 198- 200" C., and whose critical pressure, Pc, is 620640 p.s.i.g. The liquid solvent is pumped through the supply line to the solvent pre-heatcr which raises the temperature of the solvent by means of a hot water jacket to 50 C., the solvent pressure then being p.s.i.g. The reciprocating solvent pump meters the solvent at the rate of about 148 lbs/hr. through a high pressure line to a steam jacketed heater which raises the temperature to about 150 C., the total pressure now being about 1800 p.s.i.g. The solvent autogenous pressure at 150 C. is 295 p.s.i.g. The hot high pressure solvent passes then through the high pressure transfer line to the screw mixer.

In the screw mixer hot trichlorofluoromethane is injected into the stream of molten polymer and a solution is formed as the material passes along the screw which turns at 35 rpm. The solution which passes from the screw mixer contains 14% by weight polymer and is formed at a rate of approximately 175 1bs./hr. in the screw mixer. The temperature of the eflluent from the mixer is maintained at 185 C. by control of the jacket temperature in the solvent heater. The blanket around the screw mixer balances heat lost from the mixer. The pressure of the effluent solution is about 1720 p.s.i.g. Since the autogeneous pressure of the solvent at 185 C. is 515 p.s.i.g. it is apparent that considerable pressure has been imparted to the solution by the screw extruder and solvent pump. The solution apparent Newtonian viscosity is 35 poises (measured at 185 C.).

From the screw mixer the solution passes through a transfer line into an averaging mixer which has a high shear zone at the point of solution entry followed by a low shear zone with a hold-up of about 43 gals. for developing a solution of uniform concentration. The shearing action is developed by screw flights on a shaft which is centered in the cylindrical entry at the bottom of the averaging tank. A common shaft is used for the screw flight in the high shear zone and for the paddles which stir the material in the low shear zone. The high shear zone is inches in length and has a diameter of 1.5 in. The screw flight has a pitch of 0.25 inch per revolution, and the channel depth between flights is 0.12 inch, and the clearance from screw flight to cylinder wall is 0.25 inch. The shaft turns at 134 revolutions/minute. The temperature of the solution in the averaging tank is maintained at 185 C. The pressure is about 1650 p.s.i.g. at the time of start up.

The solution proceeds from the transfer line to two filters in parallel. The filters are cylindrical in shape, inches long, 2 /2" diameter, and will retain particles greater than .006 inch in diameter.

The solution passes from the filters to a solution transfer line which contains a pressure sensor, specifically a Swartwout type P3T/2 pressure transmitter and Heise pressure gauge. Pressure is transmitted by the transmitter to a type A8C/ 4 Swartwout pressure recorder and controller which in turn supplies an electronic signal to a Reliance D.C. drive system which controls the speed of drive motors for the melt extruder and solvent pump.

The pressure in the transfer zone downstream from the filters is maintained in this example at 1640 p.s.i.g. -12 p.s.i.g. and the temperature in the transfer Zone is maintained at 185 C. :1 C.

The two-liquid boundary pressure for the solution at 185 C. is 1245 p.s.i.g.

The solution passes from the filters to an automatic letdown valve which serves as the gateway in the flow line to the spinning zone, which is controlled at a lower pressure by a second pressure control loop. The pressure downstream from the valve constriction is controlled by the amount of valve opening. The valve opening is controlled by means of a pressure transmitter (Taylor type X726TN) which controls the pneumatic output of a pressure controller (Taylor type 704RE) which in turn regulates the amount of valve opening. From the automatic valve the solution passes through a pipe to a single spinneret orifice, the volume of solution in the pipe being about 100 cc. In this experiment there is no appreciable pressure let-down within the spinneret itself up to the point of extrusion. The orifice has a diameter of 0.025 inch and a passage length of 0.025 inch. In this experiment the automatic valve operates between 40 to 60% open at all times while pressure is controlled within the limits 1050 p.s.i.g. :12 p.s.i.g. From the orifice the solution passes to the surrounding atmosphere at .a high rate of speed whereupon solvent evaporates vigorously and a highly fibrillated strand forms. The spinning strand impinges on a deflector placed just outside the spinneret orifice which causes the strand to form an open network which is laid in overlapping layers upon a moving belt.

The loose material on the belt passes then between a pair of rolls which exert a light pressure of 10 lbs./ linear inch along the roll axis. A sheet product is obtained having a tensile strength of 12 lbs./in.//oz./yd

The product made by the apparatus and process of this invention is particularly well adapted for use in wall covering, in tarpaulins, in cigarette filters and in other uses where uniform density and uniform covering power are important.

Throughout an operation over an extended period of time no variations in the speed of plexifilament production are apparent. The pressures of 1640 p.s.i.g. :12 p.s.i.g. in the transfer zone and 1050 :12 p.s.i.g. in the let-down are consistently maintained by controlled adjustments of the drives of extruder 42 and solvent pump 61 while the temperature remains constant. Even as the filters become partially clogged, those pressures remain constant.

A sample of the strand is collected as it comes off of the deflector below the spinneret. The strand itself is found to have uniform morphology along its length and a uniform denier of 300 to 350.

Fibril count measurements are obtained by freezing the strand in ice and cutting cross-sections 0.1 mm. thick. After thawing, individual particles within the out are counted to obtain an indication of fibrillation. There are at least 50 fibrils/1000 denier/0.1 mm. at every point along the length of the strand. For purposes of comparison strands made by a technique employing a constant solution throughput without concern to fluctuations in spinning pressure are found to contain many solid particles and areas of low fibril count. Evidently, pressure fluctuations of such magnitude were encountered that premature formation of a two-phase system developed. Many portions of the resulting strand have a fibril count of only about 1 fibril/mm./ 1000 denier/0.1 mm.

EXAMPLE H The system described in Example I is altered by eliminating the let-down valve 55 and let-down pressure sensor element 69 and control unit 70 shown in FIGURE 3. These are replaced by a spinneret of the type shown in FIGURE 2 having two constrictions, the upstream one 22 having a diameter of .032 inch and a length .032 inch and the downstream one 24 having a diameter of .025 inch and a length of .025 inch. The small space 23 defined by the two has a volume of 6 cc. and acts as a pressure let-down chamber since there is no separate let-down pressure control unit. A deflector is placed opposite the spinneret hole as in Example I.

Using the same polymer and solvent and the same dissolving Zone, transfer zone, temperature, pressure, and concentrations as in Example I, a linear polyethylene strand is formed having a high degree of fibrillation continuously along its length. Comparative strands produced using a constant flow rate have many unfibrillated portions.

EXAMPLE III The apparatus described in Example I is modified to spin five strands by replacing the single automatic valve 55' of FIGURE 3 with a manifold leading to five separate spinnerets each controlled by an automatic valve. A flow diagram of the manifold, automatic valves, pressure letdown controls and spinnerets is shown in FIGURE 4. In FIGURE 4 transfer line 50, dual filters 54, pressure sensor element 51, and transfer line are the same as in FIG- URE 3. In this case however, the transfer line 89 connects with a manifold 85, which is designed so as to distribute the solution to legs 86 with approximately equivalent pressure drops in each legs. The solution in each leg passes through an automatic valve 87 to a spinneret feed line 88 having a pressure sensor element 89 which feeds a signal to a pressure control unit 90 which in turn regulates the opening in the automatic valve 87. The spinneret feed line following each valve line delivers solu- 19 tion to a spinneret assembly 91 having a spinning orifice .025 inch in diameter and a passage .025 inch long.

In operating the process of this example a 14% solution of linear polyethylene is continuously formed as in Example I. The pressure in the solution transfer line 80 of FIGURE 4 is maintained at 1450:12 p.s.i., a pressure sensor 51 transmits a signal to the speed level and pressure controller just as in Example I. This controller regulates the overall speeds of the screw extruder supplying polymer and the solvent pump. At the same time it regulates the speed ratio of the polymer and solvent pumps.

Each automatic valve 87 serves as a gate to a region of lower pressure 88 already identified as the pressure letdown zone. The pressure let-down sensor elements and pressure let-down control units 90 by regulating the automatic valve 87 maintain the pressure in each of the letdown zones at 1200: p.s.i. In order to maintain this pressure, the pressure sensor element 51 of FIGURE 4 calls for a much greater flow of solution than was needed in Example I. At equilibrium this results in about 120 lbs/hr. of linear polyethylene polymer and about 740 lbs. of trichlorofluoromethane being delivered to the dissolving zone.

From the five spinnerets, five separate strands 92 are obtained.

Oscillating deflectors (not shown) are placed opposite each spinneret to spread the fibrillated strand to an open web.

Samples are collected from the five deflectors below the spinnerets and then a single sheet is collected on a moving belt from the five traversing networks. After light pressing between two rolls with a pressure of 10 lbs./ linear inch the sheet has a tensile strength of 12 lbs./in.//oz./yd. tear strength of 1 lb.//oz./yd. and a density of 9 lbs./ft.

Each of the five strand networks has a uniform appearance and a high degree of fibrillation throughout its length.

In this five spinneret experiment, one of the spinnerets may be shut down without the morphology of the other strands changing. Pressure in transfer line 80 is still maintained at 1450 p.s.i. :12. When one spinneret is shut down the total amount of polymer and solvent delivered drops automatically to about 96 and about 592 lbs/hour respectively.

EXAMPLE IV The procedure of Example I is repeated employing essentially the same apparatus except that the single position spinning zone is replaced with a 3-position manifold, each line thereof containing both an automatic valve, as shown in FIGURE 4, and a spinneret of the type shown in FIGURE 2. The polymer and solvent are the same as in Example I. The plexifilamentary material discharged from the three spinning orifices is collected and formed into a coherent sheet material.

Various characteristics and conditions of the operation are as follows:

Rate of polymer feed lbs./hr 50 Screw extruder temperature:

1st zone C 175 2nd zone C 205 3rd zone C 205 Polymer temperature entering the screw mixer C 220 Solvent preheater:

Temperature C. 40 Pressure p.s.i.g 125 Solvent pressure after piston pump p.s.i.g 1900 Solvent temperature after leaving solvent heater C 185 Screw mixer temperature C 185 Solution concentration perccnt 13.5 Averaging mixer hold-up time min 50 Pressure at the averaging mixer discharge line p.s.i.g 1800 20 Polymer flow rate (3 positions) Total g./min 375 Per postion g./min Manifold lines (upstream of auto. valve) Temperature C Pressure p.s.i.g 1650 Pressure let-down (between auto valve and spinneret):

Temperature C 185 Pressure p.s.i.g 1250 Spinneret, first orifice:

Diameter in 0.028 Length in 0.029 Chamber volume cc 6 Final orifice:

Diameter in 0.020 Length in 0.025

The discharged plexifilamentary material from each spinning orifice is caused to impinge upon rapidly oscillating baffies disposed at an angle of 35 and electrostatically deposited in overlapping layers upon a single moving belt having a surface speed of 9.5 feet per minute. The resultant continuous length of sheet material, width 30% inches, is lightly compressed by the action of a pair of pressure rolls exerting a nip pressure of 14 lbs/linear inch and is wound into rolls under a tension of 0.1 lb./ in. width. Basis weight of the product is 2.3 oz./yd.

The three discharging plexifilamentary strands are found to have a remarkably uniform morphology, including essentially constant denier, throughout an extended period of operation.

In the foregoing examples the pressure reading of greatest importance is that immediately following the solution filters 54 and preceding the automatic let-down valve (55 Example I) or the spinneret pressure let-down chamber (23 of Example l1). This pressure, identified as the transfer zone pressure, is indicated by the pressure transmitter 51 or by a separate gauge in association therewith. Pressures upstream from the transfer zone will of course be somewhat higher.

The temperature of the solution in the flow line preceding the pressure let-down spinneret or automatic pressure let-down valve is identified as the transfer zone temperature. The primary heat sensor is located in the solution transfer line 47 which immediately follows the screw mixer. All succeeding temperatures up to the pressure let-down device are made to conform with this temperature. Losses in heat are balanced by means of heating elements at various points along the process line since the maintenance of a constant temperature is essential to the maintenance of a constant pressure.

EXAMPLE V A single continuous strand of highly fibrillated linear polyethylene is spun using the apparatus and process shown in FIGURE 6. A pressure control loop maintains pressure in the transfer zone upstream of the letdown zone. A second pressure control loop controls the pressure in the letdown zone immediately following an automatic valve construction. The spinneret assembly has a constriction at the letdown orifice and at the final orifice. The final orifice has a diameter of 0.030 inch and a passage length of 0.035 inch.

Various components of the equipment in the flow diagram will now be described in detail.

The cold slurry is formed in a tank 109 by feeding polymer and solvent in a predetermined ratio and by agitating the slurry using a Lightnin mixer model No. NLD-200. Brine at 7 C. is passed through cooling coils around the tank to maintain the solvent at a temperature lower than its boiling point.

In operating the system linear polyethylene of melt index 0.5 having a density of 0.953 g./cm. is fed in ground 21 particle form (finer than 30 mesh) into the slurry tank at a rate of about 48 lbs./ hr.

Solvent for the system is supplied from a lower pressure storage tank. The solvent is trichlorofluoromethane which boils at 24 C., whose critical temperature Tc is 198200 C. and whose critical pressure, P0, is 620-640 p.s.i.g. The liquid solvent is metered into the cold slurry tank at the rate of about 296 lbs/hr.

The cold slurry is passed through a pipe from the cold slurry tank to the slurry pump 115. The slurry pump is De Lavals Hydropulse pump model H-3. This is a double diaphragm positive displacement pump with ball check valves. A lubricating oil is the pumping medium. The oil is raised to a pressure of approximately 1900 p.s.i.g. by means of a positive displacement constant speed pump. A phasing valve determines which diaphragm is pressurized by the oil and which is drained. The high pressure oil causes the diaphragm to enlarge pushing out an element of slurry. When the phasing valve changes, the diaphragm returns to its original size and in doing so, draws in a new element of slurry. The pump produces a cold slurry at about 15 C. and 1850 p.s.i.g.iSO p.s.i.g. Pressure fluctuations arising from the pulsating nature of the pump are partially dampened out by a Greer Hydraulics Inc. accumulator model No. 30A-1/ 4A on the high pressure oil. The pump is provided with an automatic valve, not shown, to compensate for any fluctuations in pressure that might be encountered as the polymer is dissolved and forwarded to the spinneret assembly. Thus as the opening of the automatic valve is increased, a greater quantity of oil is caused to bypass the phasing valve and diaphragm and as a result pressure on the slurry is lowered. On the other hand, a decrease in the size of the opening will raise the pressure on the slurry.

The high pressure cold slurry is passed through a slurry heater 121. The pipe 120 leading from the pump to the heater and the heater itself are both designed to provide turbulent flow mixing. Density differences between the solvent and the polymer will cause separation if mixing is not maintained. The tubing size for these elements is 0.3125 inch O.D. tubing having an ID. of 0.198 inch. Twenty feet of tubing are required for the heater. This heater tubing is coiled on a 4" diameter pipe and placed inside a steam jacket, made of 6 pipe, 20" long, and maintained at approximately 220 C. giving an efiiuent temperature of 185 C.- 1 C.

The eifiuent from the slurry heater is passed into the four-stage mixer 123 with a holdup of 38 gallons. Staging is provided by baflies 127 and each stage has a paddle mixer. All four paddles 125 are on a single driven shaft. The temperature of the solution in the mixer is maintained at 185 C. The material passing into the mixer is a hot slurry and dissolving occurs within the mixer. The material coming out of the mixer is a uniform solution.

The solution proceeds from the dissolving zone into the transfer zone and pressure letdown zones which are identical to those described in Example I above.

The pressure in the transfer zone downstream from the filters 54 is maintained in this example at 1640 p.s.i.g. :12 p.s.i.g. Pressure in the zone immediately following the automatic valve is maintained at 1360: p.s.i.g. The pressure control is obtained on pressure signals obtained from a Taylor button bulb and transmitter 51 (Taylor type X726TN). These control the output of a pressure controller 68 (Taylor type 704RE which by means of electric-to-air transducer 107 directs appropriate changes in the output of the pump; i.e. by changing the opening of the automatic valve. The pressure in the chamber between the first and second spinneret constrictions is about 1000 p.s.i.g.

The temperature in the transfer zone is maintained at 185 C.i1 C. by a feedback loop which regulates the pressure of the steam in the jacket of the slurry heater 121.

The morphology of the plexifilamentary product so obtained is similar in terms of uniformity to that of the product of Example 1.

EXAMPLE VI The apparatus described with reference to Example V is again employed except that several interconnected lengths of steam-jacketed pipe of varying diameter replace the mixer 123. Process conditions are also the same except for an increased throughput rate. Thus suflicient polymer and solvent is charged to the cold slurry tank 109 to provide a slurry containing 13% polymer. The slurry is discharged from the slurry pump 121 at a rate of 60 lbs. polymer/hr. For the initial pipe section immediately following the slurry heater, 11 feet of a 1" nominal diameter schedule pipe is used. The holdup time of this length is 0.4 minute. Following this section is 8' to 2 nominal diameter double extra strong pipe. The holdup time in this section is 0.8 minute. The flow through these first two sections i vertically upward. The next section is a horizontal section of 2" nominal diameter double extra strong pipe 18 long. Holdup time in this section is 1.9 minutes. Flow through the next section, which consists of an 8' section of 4" nominal diameter double extra strong ipe, is vertically downward. The holdup time of this section is 13 minutes. Additional elements such as the filters provide further holdup time. The total holdup time from slurry pump to spinneret orifice is 18 minutes. Thorough mixing or agitation of the solvent and polymer is effected by the turbulent flow which is encountered in the heater 121. Laminar flow within the pipe sections further ensures uniform contact of polymer and solvent.

Throughout 19 hours continuous operation, the morphology of the plexifilaments so obtained was exceptionally uniform.

EXAMPLE VII The system described in Example V is altered by eliminating the mixer 123, replacing the single-stage slurry heater 121 with a two-stage slurry heater, and by replacing the single spinneret with two spinnerets. A throughput rate of 43 lbs/hr. of polymer (in a 13.2% slurry) is obtained. The first spinneret has a diameter of 0.020 inch and a length of 0.020 inch. The second spinneret has a diameter of 0.030 inch and a length of 0.035 inch. The respective throughput rates are 17 lbs./hr. of polymer from the first spinneret and 31 lbs/hr. of polymer from the second.

The first section of the two-stage slurry heater which comprises the dissolution system is a coiled tubing, 20 feet in length, having a 0.3125 inch nominal OD. and a ID. of 0.198 inch. This tubing, providing a 3 seconds holdup time, is coiled in a 4 inch diameter helix and is contained in a steam jacket made of 6" pipe, 20 long which is maintained at 115 C. The efiluent from this rst section of the heater is a warm slurry at 110 C. Flow in this first section is turbulent and provides agitation for the dissolution system.

The warm slurry which is the effluent of the first section of the slurry heat exchanger is not a slurry of polymer particles and solvent. Rather the polymer particles have imbided most of the available solvent and have become swollen. Thus the thoroughly mixed effiuent of the first section of the heater is a mass of swollen polymer particles. This mass is close-packed and i handled in laminar flow without loss of concentration uniformity.

The second section of the slurry heater is made of 0.625 inch nominal O.D tubing with ID of 0.435 and 113 feet long. This tubing is coiled in an 8" diameter helix and is contained in a 12 in. diameter, 30" long pipe in a steam jacket which is maintained at about 220 C. The efiiuent of this heater section is a solution at C. Upon heating from 110 C., the warm slurry be comes a uniform solution.

The temperature in the transfer zone is maintained'at 185 C. Ll C. by a feedback loop which regulates the pressure of the steam in the jacket of the second section of the slurry heater.

The pressure in the transfer zone at the sensor 51 is maintained as in Example V at 1640 p.s.i.g. :12 p.s.i.g. Because of the higher pressured drop in thet slurry heater, the slurry pump operates at a pressure of about 1200 p.s.1.g.

While the system produces uniform morphology when the transfer zone pressure is kept with :50 p.s.i., still greater uniformity is achieved if the transfer zone pressure is consistently kept with :15 psi. at a point in the flow line immediately preceding the let-down zone.

The process and equipment described here may be used to prepare a variety of fibrous products ranging from continuous strands to loose discontinuous fluffs.

The uniformity of the products of this invention is important in many field of manufacture, giving cigarette filters of constant filtering power, sheet products such as tarpaulins with uniform strength and light non-woven textiles with uniform optical cover.

Terminology herein with respect to spinning, strand material and derivative words thereof, while primarily intended to connote the extrusion of continuous threadlike filaments, is nevertheless to be understood as also including extrudates of other continuous elongated shapes, e.g. sheet structures wherein one cross-sectional dimension greatly exceeds the other and tubular shape products.

As many widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that this invention is not to be limited to the specific embodiments thereof except as defined in the appended claims.

What is claimed is:

1. A continuous process for the flash spinning of fibrillated plexifilamentary material by the steps of continuously supplying under pressure into a dissolution zone molten, synthetic crystallizable, oragnic polymer of filament-forming molecular weight and a solvent for the polymer, the concentration of polymer being 2 to by weight of the solution, dissolving the polymer and forming a polymer solution having a temperature of at least the solvent critical temperature minus 45 C. and a pressure above the two-liquid-phase pressure boundary for the solution, forwarding the solution through a transfer zone, passing the solution into a pressure let-down zone for lowering the pressure of the solution to below the two-liquid-phase pressure boundary for the solution, maintaining a constant pressure in the transfer zone preceding the pressure let-down zone by keeping the heat balance in the transfer zone at substantially the same level as in the dissolution zone and by regulating the supplies of polymer and solvent to the dissolution zone inversely in relation to the pressure in the transfer zone and in a fixed ratio to one another, and discharging the solution thorugh a spinneret orifice of restricted size to an area of substantially atmospheric pressure and temperature.

2. The process of claim 1 wherein the polymer is a crystalline polyhydrocarbon.

3. The process of claim 1 wherein the polymer is polyethylene.

4. Process of claim 1 wherein the solvent has a boiling point at least C. below the melting point of the polymer and dissolves less than 1% by weight of the polymer at that boiling point.

5. Process of claim 1 wherein the let-down zone lowers the pressure of the solution to no less than the autogenous pressure of the solvent at the critical temperature minus vC.

6. Apparatus for continuously producing strand material from a synthetic crystalline organic polymer comprising: feed devices for advancing respectively said polymer in fluid form and a heated solvent therefor under pressure into a dissolution chamber having an agitation means and providing a stream of polymer solution, flash spinning apparatus comprising a spinneret assembly, said spinneret assembly comprising structure which defines a spinning orifice and a constriction for lowering the pressure of the stream upon flow therethrough prior to discharge through the spinning orifice, a flow line operatively connecting the dissolution chamber to the spinning orifice, means for maintaining a constant temperature within the flow line between the dissolution chamber and the constriction, pressure control means for inversely varying in a fixed ratio the output of said feed devices dependent on the pressure in the flow line, and a pressure sensor coupled with the flow line between the dissolution chamber and the constriction, said pressure sensor being connected to said pressure control means for transmitting a pressure indication thereto.

7. Apparatus of claim 6 wherein said polymer feed device is a screw extrusion device for producing a molten flow of polymer.

8. Apparatus of claim 6 wherein said means for maintaining a constant temperature includes a temperature control element for inversely varying the temperature of solvent from the solvent feed device relative to the temperature in the flow line, and a temperature sensor within the flow line between the dissolution chamber and the constriction, said temperature sensor being connected to said control means for transmitting a temperature indication thereto.

9. Apparatus of claim 6 wherein said flash spinning apparatus comprises a plurality of said spinneret assemblies in parallel arrangement.

10. Apparatus of claim 6 wherein a filter device for the stream is within the flow line between the dissolution chamber and said pressure sensor.

11. Apparatus of claim 6 wherein said constriction is an inlet orifice of the spinneret assembly.

12. Apparatus of claim 6 wherein said constriction is a valve in the spinneret assembly for adjusting to a selected pressure lowering.

13. Apparatus of claim 12 wherein said spinneret assembly comprises a second pressure control means for inversely varying the flow through said valve dependent on the pressure in the flow line between the constriction and the spinning orifice, and a second pressure sensor coupled with the flow line between the constriction and the spinning orifice, said second pressure sensor being connected to said second pressure control means for transmitting a pressure indication thereto.

14. Apparatus of claim 13 wherein said flash spinning apparatus comprises a plurality of said spinneret assemblies in parallel arrangement.

15. A continuous process for the flash spinning of fibrillated plexifilamentary material by the steps of continuously supplying under pressure into a dissolution zone, synthetic crystallizable, organic polymer of filamentforming molecular weight and a solvent for the polymer, the concentration of polymer being 2 to 20% by weight of the solution, dissolving the polymer and forming a polymer solution having a temperature of at least the solvent critical temperature minus 45 C. and a pressure above the two-liquid-phase pressure boundary for the solution, forwarding the solution through a transfer zone, passing the solution into a pressure let-down zone for lowering the pressure of the solution to below the two-liquid-phase pressure boundary for the solution, maintaining a constant pressure in the transfer zone preceding the pressure let-down zone by keeping the heat balance in the transfer zone at substantially the same level as in the dissolution zone and by regulating the supply of polymer and solvent to the dissolution zone inversely in relation to the pressure in the transfer zone and in a fixed ratio to one another, and discharging the solution through a spinneret orifice of restricted size to an area of substantially atmospheric pressure and temperature.

16. Process of claim 15 wherein said polymer and said solvent are supplied in the form of a slurry to said dissolution zone.

17. Process of claim 16 wherein said polymer is linear polyethylene and said solvent is trichlorofiuoromethane.

18. Apparatus for continuously producing strand material from a synthetic crystalline organic polymer comprising: feed device means for advancing said polymer and a solvent therefor at a fixed ratio under pressure to a dissolution system having agitation means and providing a stream of heated polymer solution, flash spinning apparatus comprising a spinneret assembly, said spinneret assembly comprising structure which defines a spinning orifice and a constriction for lowering the pressure of the stream upon fiow therethrough prior to discharge through the spinning orifice, a flow line operatively connecting the dissolution system to the spinning orifice, means for maintaining a constant temperature within the flow line between the dissolution system and the constriction, pressure control means for inversely varying the output of said feed device means dependent on the pressure in the flow line, and a pressure sensor coupled with the flow line between the dissolution system and the constriction, said pressure sensor being connected to said pressure control means for transmitting a pressure indication thereto.

19. Apparatus of claim 18 wherein said feed device means comprises a pump and operatively connected thereto a supply vessel for a slurry of said polymer in said solvent.

20. Apparatus of claim 18 wherein said flash spinning apparatus comprises a plurality of said spinneret assemblies in parallel arrangement.

No references cited.

ALEXANDER H. BRODMERKEL, Primary Examiner. 

1. A CONTINUOUS PROCESS FOR THE FLASH SPINNING OF FIBRILLATED PLEXIFILAMENTARY MATERIAL BY THE STEPS OF CONTINUOUSLY SUPPLYING UNDER PRESSURE INTO A DISSOLUTION ZONE MOLTEN, SYNTHETIC CRYSTALLIZABLE, ORAGNIC POLYMER OF FILAMENT-FORMING MOLECULAR WEIGHT AND A SOLVENT FOR THE POLYMER, THE CONCENTRATION OF POLYMER BEING 2 TO 20% BY WEIGHT OF THE SOLUTION, DISSOLVING THE POLYMER AND FORMING A POLYMER SOLUTION HAVING A TEMPERATURE OF AT LEAST THE SOLVENT CRITICAL TEMPERATURES MINUS 45*C. AND A PRESSURE ABOVE THE TWO-LIQUID-PHASE PRESSURE BOUNDARY FOR THE SOLUTION, FORWARDING THE SOLUTION THROUGH A TRANSFER ZONE, PASSING THE SOLUTION INTO A PRESSURE LET-DOWN ZONE FOR LOWERING THE PRESSURE OF THE SOLUTION TO BELOW THE TWO-LIQUID-PHASE PRESSURE BOUNDARY FOR THE SOLUTION, MAINTAINING A CONSTANT PRESSURE IN THE TRANSFER ZONE PRECEDING THE PRESSURE LET-DOWN ZONE BY KEEPING THE HEAT BALANCE IN THE TRANSFER ZONE AT SUBSTANTIALLY THE SAME LEVEL AS IN THE DISSOLUTION ZONE AND BY REGULATING THE SUPPLIES OF POLYMER AND SOLVENT TO THE DISSOLUTION ZONE INVERSELY IN RELATION TO THE PRESSURE IN THE TRANSFER ZONE AND IN A FIXED RATIO TO ONE ANOTHER, AND DISCHARGING THE SOLUTION THOROUGH A SPINNERET ORIFICE OF RESTRICTED SIZE TO AN AREA OF SUBSTANTIALLY ATMOSPHERIC PRESSURE AND TEMPERATURE.
 6. APPARATUS FOR CONTINUOUSLY PRODUCING STRAND MATERIAL FROM A SYNTHETIC CRYSTALLINE ORGANIC POLYMER COMPRISING: FEED DEVICES FOR ADVANCING RESPECTIVELY SAID POLYMER IN FLUID FORM AND A HEATED SOLVENT THEREFOR UNDER PRESSURE INTO A DISSOLUTION CHAMBER HAVING AN AGITATION MEANS AND PROVIDING A STREAM OF POLYMER SOLUTION, SLASH SPINNING APPARATUS COMPRISING A SPINNERET ASSEMBLY, SAID SPINNERET ASSEMBLY COMPRISING STRUCTURE WHICH DEFINES A SPINNING ORIFICE AND A CONSTRICTION FOR LOWERING THE PRESSURE OF THE STREAM UPON FLOW THERRETHROUGH PRIOR TO DISCHARGE THROUGH THE SPINNING ORIFICE, A FLOW LINE OPERATIVELY CONNECTING THE DISSOLUTION CHAMBER TO THE SPINNING ORIFICE, MEANS FOR MAINTAINING A CONSTANT TEMPERATURE WITHIN THE FLOW LINE BETWEEN THE DISSOLUTION CHAMBER AND THE CONSTRICTION, PRESSURE CONTROL MEANS FOR INVERSELY VARYING IN A FIXED RATIO THE OUTPUT OF SAID FEED DEVICES DEPENDENT ON THE PRESSURE IN THE FLOW LINE, AND A PRESSURE SENSOR COUPLED WITH THE FLOWING BETWEEN THE DISSOLUTION CHAMBER AND THE CONSTRICTION, SAID PRESSURE SENSOR BEING CONNECTED TO SAID PRESSURE CONTROL MEANS FOR TRANSMITTING A PRESSURE INDICATION THERETO. 